DESCRIPTION
The Mark VI is a vacuum tube high power basic power
amplifier designed for the professional sound installer.
Vacuum tubes are most often favored in their ability to with-
stand enormous physical and electrical abuses—conditions
frequently encountered in discotheques, public address and
musical instrument applications. Its rugged construction
and utilization of superior components, including the
patented Dynaco Super Fidelity transformers, permit con-
tinuous duty at full power, while maintaining safe operating
margins. Protective isolation of the load from the driving
circuits, and uniform power output into a variety of load
impedances are inherent in all designs which use an impe-
dance matching transformer.
The Mark VI employs a basic circuit arrangement which
all Dynaco vacuum tube amplifiers have used since the
inception of the Mark II amplifier in 1955. Refinements of
the operating parameters have been incorporated for circuit
balance, stability, and for drive capability. The components
used have been selected to protect against failure, and all
parts are operated conservatively to assure superior per-
formance and proper operation for many years.
The pentode section of the 7199 tube is a high gain voltage
amplifier, which is direct-coupled to a cathodyne—split
load—phase inverter. The operation of this type of phase
inverter is independent of tube aging, a unique advantage
of its design. The input gain and inverter stages are refined
to provide an accurately balanced signal to the output tubes.
The 8417 output tubes are a matched set of four, to assure
proper operation of the Mark VI. The output tubes are op-
erated in a fixed bias arrangement, which may be accurately
checked and adjusted with the lighted meter and recessed
potentiometer on the front panel.
The output tubes drive a Dynaco Super Fidelity output
transformer expressly designed for this circuit. Taps on the
plate windings connect to the screen grids, and contribute to
the linearity of the overall design. The impedance match
and bias conditions employed contribute to very low distor-
tion over a wide range, even without feedback. The overall
feedback loop adds 20 decibels of additional distortion re-
duction.
In addition to front panel adjustment of bias, recessed con-
trols for input level and a 4-position range switch (three
positions for observing output power levels on the meter,
plus one position for observing bias) are supplied. A speaker
fuse holder and lighted power switch complete the front
panel layout.
The simplicity of this circuit—a minimum of phase shift-
ing stages and very careful attention to output transformer
design—assures maximum stability.
INSTALLATION
The Mark VI is designed for mounting in a standard 19"
rack (mounting hardware for the rack is not supplied); its
supplied large feet also permit placement on any flat sur-
face. The amplifier should be installed where the front panel
is accessible. If the Mark VI is to be mounted near a phono-
graph, check to make sure that the phono cartridge does not
pick up hum from the power transformer.
VENTILATION
is an important consideration. This amplifier
generates almost the same heat under quiescent (no signal)
conditions as it does at full power—the equivalent of a 250
watt light bulb within the confines of the perforated cage.
Thus, ADEQUATE AIR CIRCULATION IS ESSEN-
TIAL. If the Mark VI is rack mounted, 1 foot (30 centi-
meters) of unimpeded air space
MUST
be provided above
the unit. Otherwise fan cooling is recommended. Also, the
amplifier is intended to be installed horizontally, with its
feet providing clearance for air flow under the unit. Do not
allow the ventilation slots in the bottom to be restricted
(don't place the amplifier on a rug, for example), or there
will be excessive heat buildup within the enclosed section of
the chassis. Vertical mounting is not recommended, even
with fan cooling.
If the amplifier is to be permanently installed, the han-
dles may be removed
before installation,
if desired. The
bolts which affix the handles to the front panel should be
reinstalled once the handles have been removed, since these
bolts also attach the front panel to the chassis. However, the
handles not only facilitate carrying the Mark VI, but they
also protect the front panel components from breakage.
Therefore, unless it is to be permanently mounted, their re-
moval is not suggested.
CONNECTIONS
INPUT
A signal input is connected by a single conductor shielded
cable to a choice of two
RCA
type phono jacks or an
XLR
con-
nector. One of the phono jacks and the
XLR
input provide flat
low frequency response; the other phono jack provides a low
frequency rolloff @ 6 dB per octave (—3 dB @ 70 Hz). The
input impedance is 50,000 ohms, unbalanced line. The input
sensitivity is 1.6 volts rms for 120 watts output with the input
level control turned fully clockwise. The Mark VI may
therefore be easily driven by one channel of many brands of
control preamplifiers, such as the Dynaco
PAT
-5,
PAT
-4,
PAS
-3
X
, or
PAS
-2/3. The
PAS
tube type preamplifiers require
internal modification: To change the
PAS
-3
X
, simply locate
and remove the pair of 62,000 ohm resistors (blue-red-
orange) on the
PC
-5 printed circuit board; to change the
PAS
-2 or
PAS
-3, remove the 62,000 ohm resistors, as indicated
4
above, and additionally remove the pair of 510,000 ohm re-
sistors (green-brown-yellow) on the back panel of the pre-
amplifier.
Some other tube type preamplifier brands may require
modification for optimum performance into a 50,000 ohms
load.
OUTPUT
Select speaker leads of sufficient size to preserve the
damping factor of your amplifier. Standard 18 gauge lamp
cord ("zip cord") is suitable for distances up to 30 feet.
Larger wire sizes are advised for longer distances—#16 for
50 feet; #14 for 80 feet. #18 and #16 lamp cord is available
from hardware and electrical supply stores, and is the
easiest to use and conceal.
